I'm not sure that all the cars are ex rental, but I've never dealt with them.
Hertz Ireland also have a fleet leasing business. They tend to keep their hire cars for a very short time to avoid any issues with maintenance or breakdown when out on hire. It's not uncommon for hire companies to only keep cars for a single season, selling the majority of cars at about 9 months before the first service is due.
The older cars are more likely ex-lease, or sourced elsewhere.
Remember, Hertz Ireland is part of the Boland group, so they've plenty of supply of used cars.